How do the number of electrons in an ion of an element differ from those of the non-ionic atoms of that element?  

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Ions are formed when atoms gain or lose electron/s. The atoms that do not participate in this process, thus have either more or less electrons than ions. Let us understand this with an example. Sodium is a metal and has an atomic number of 11. Thus, sodium atoms have 11 electrons. It has an electronic configuration of `1s^2, 2s^2, 2p^6, 3s^1` . To form an ion, sodium loses the lone 3s electron and becomes a cation, `Na^+` . The cation has only 10 electrons, as compared to a sodium atom that has 11 electrons. Similarly, chlorine has 17 electrons in non-ionized state. A chloride ion, `Cl^-` , is formed when chlorine atom gains 1 additional electron. Thus, a chloride ion has 18 electrons, while a chlorine atom has 17 electrons.

Thus, the electron gain or loss causes the ions to have more or less electrons as compared to regular atoms of the same element.

Hope this helps. 

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